Thermoforming or vacuum forming is essentially taking a piece of thermoplastic sheet, heating it to a point beyond its softening point, vacuuming it to a contoured surface, and cooling the formed sheet back down to below its softening point. A large variety of thermoplastic materials can be subjected to this treatment. Some of the common materials used in thermoforming are listed as follows:
High Impact Polystyrene (HIPS) PA1 Acrylonitrile-Butadiene-Styrene (ABS) PA1 Styrene-Acrylonitrile (SAN) PA1 Polyvinyl Chloride (PVC) PA1 Polycarbonate (PC) PA1 High Density Polyethylene (HDPE) PA1 Polyphenylene Oxide (PPO) This unique forming technique has also been successfully used to laminate decorative expandable vinyl cover materials onto contoured plastic or fiberboard substrates by using a special class of adhesive, called thermoforming adhesives (see, e.g. Chan and Holnisch, Adhesives Age, February 1986, pp 18-20). Typically, the adhesive is first applied to the contoured part by spray, and then dried. The vinyl is then thermoformed onto this adhesive coated surface. This process has been widely used for producing high quality, domestically appealing parts for the automotive industry and furniture trades. Some typical applications include thermoforming PVC film in the manufacture or automobile consoles, dashboards, door panels and other interior trim as well as in the production of decorative furniture facings.
A variety of adhesives can be used in this process including both organic solvent and water-based adhesives. However, in general, solvent-based adhesives are used more extensively since such systems cure faster, exhibit higher cured bonding strengths and exhibit higher "green strengths" than their water-based counterparts. "Green strength" as used herein is the ability of the adhesive to hold two surfaces together when first contacted and before the adhesive develops it ultimate bonding properties when fully cured. The degree of green strength exhibited by an adhesive is very important in many applications, including thermoforming. High green strength adhesives tend to prevent wrinkling and slippage of films during lamination.
For these reasons, the (organic) solvent-based adhesives are the standard adhesives used in the thermoforming industry. These solvents, however, are often flammable and/or toxic. As the regulation of workplace safety by federal, state, and local authorities become increasingly stringent, it will become difficult, if not impossible, to use such adhesives without costly vapor collection and containment systems.
What is needed is an adhesive which possess the desirable properties of the solvent-based systems (high cured bonding strength and high green strength) yet, which does not exhibit the drawback of toxicity and flammability commonly encountered with the use of organic solvents.